1. Field of the Invention
The present invention generally relates to video signal processing circuits, and particularly relates to a video signal processing circuit which performs encoding or the like of a video signal into which data such as caption information is inserted.
2. Description of the Related Art
Television signals these days include caption information such as closed captions, text broadcasting information, program listing information, copy guard signals, etc. Such information and signals are inserted into a plurality of horizontal lines located at the top of image fields, which include top fields and bottom fields in the case of video signals used in the interlace system, for example.
FIGS. 1A and 1B are drawings showing each field of the interlace system of the NTSC system and the position where text information or the like is inserted.
FIG. 1A shows a top field, and FIG. 1B shows a bottom field. As shown in FIG. 1A, the top field is comprised of horizontal scan lines from Line 1 to Line 263. Among the 263 horizontal scan lines, a portion from Line 1 to Line 21 at the top of the field is a blanking interval, which corresponds to a vertical blanking interval, and is not displayed on the screen. A vertical synchronizing pulse is inserted into this interval. The blanking interval may also include test signals, caption information, text broadcasting information, program listing information, copy guard signals, etc., inserted thereinto. By the same token, the bottom field shown in FIG. 1B is comprised of horizontal scan lines from Line 264 to Line 525. Among these, a portion from Line 264 to Line 284 at the top of the field is a blanking interval, into which caption information or the like may be inserted.
Text data used in the closed caption system is inserted, as 16-bit data for each field, into Line 21 of the top filed and Line 284 of the bottom field. Among the 16 bits, 2 bits are used as parity bits. Text data that may be one character for each frame may be extracted and put together to form a caption comprised of a plurality of characters, which is then superimposed on video information for the period specified by control information, thereby producing a movie subtitle or the like.
Video-and-audio signals supplied as analog signals through television broadcast, for example, may be recorded as digital signals. In such a case, the video-and-audio signals are converted into digital signals by a NTSC decoder and audio DAC (digital-to-analog converter). The converted digital signals are supplied to an MPEG2 codec for coding so as to be compressed into an MPEG2PS-system or MPEG2TS-system stream. The compressed stream is output from a port dedicated for stream of the MPEG2 codec, and is transferred by a CPU through DMA to a recording device for storage therein.
In so doing, the MPEG2 codec extracts text data inserted into Line 21 and Line 284 of the video frame coded by the MPEG2 codec, and inserts the extracted text data into the stream.
In general, the clock frequency of digital data output from an NTSC decoder (i.e., the speed at which the analog signals are supplied) is slightly different from the clock frequency with which the MPEG2 codec operates (i.e., the speed at which coding is performed). In order to absorb such a difference between the speed of data supply and the speed of coding, provision is made such that the digital data of each frame output from the NTSC decoder is temporarily stored in a buffer, and the frame data are then successively read from the buffer for coding. With the provision of the buffer, the MPEG2 codec may skip a frame or repeat a frame as such need arises at the time of coding so as to absorb the difference between the speed of data supply and the speed of coding.
In this manner, the speed difference is absorbed. Since text data or the like are inserted into each field of a video signal sequentially, however, the skipping or repeating of a frame results in the data being lost, or the same data being repeated twice. That is, if repeating is done, a caption intended to be “TEST” may become “TEEST” because of repetition of the same data occurring twice. If skipping is done, a caption intended to be “TEST” may become “TET” because of the loss of certain data.
In consideration of this, it is desirable to absorb a difference between the speed of signal supply and the speed of signal processing without undermining consistency of text information or the like at the time of processing a video signal in which the text information or the like is inserted into each frame or into each field.
[Patent Document 1] Japanese Patent Application PCT-based Publication No. 9-501808
[Patent Document 2] Japanese Patent Application Publication No. 2001-24983
[Patent Document 3] Japanese Patent Application Publication No. 2001-69439
[Patent Document 2] Japanese Patent Application Publication No. 11-75113